The invention concerns a drive system for an all-wheel driven motor vehicle having a first and a second driven axle, which drive system produces the driving connection to the wheels of the second driven axle and has a differential-speed-dependent clutch for driving the second driven axle, and an at least partially lockable differential between the wheels of the second driven axle, the differential-speed-dependent clutch comprising a pump which, when a differential speed occurs, produces a pressure which leads to the transmission of a torque to the second driven axle.
Drive systems of this type are used in all-wheel driven motor vehicles, in which one axle is driven in a continuous and slip-free manner and the other is driven via a differential-speed-dependent clutch. An (in particular lockable) inter-axle differential is thus rendered superfluous because the connection between the axles is not rigid. When the wheels of the continuously driven axle are spinning, the differential-speed-dependent clutch is used to transmit a larger torque to the second driven axle and, in an extreme case, the connection between the two axles is rigid. The second axle needs, for its part, a device for at least partially synchronizing its wheels, whether by selective braking or whether by a lockable axle differential. This is intended to prevent a wheel from spinning.
Known drive systems of this type have, as a differential-speed-dependent clutch, fluid friction clutches (viscous clutches), which may be controllable, or clutches having positive displacement pumps, as described, for example, in EP 629 790 B or in AT GM 3832. In the case of clutches having a pump, the fluid which is pressurized by the pump or the pumps can either directly transmit a torque or can act upon a friction clutch which then transmits a torque, which is dependent on the pressure which is present, to the second driven axle.
As devices for the total or partial locking of the differential between the wheels of the second axle, positive and friction systems are known which, for actuation purposes, require their own power source, whether it be mechanical, hydraulic, electromechanical or else simply their own pump. In the case of positive systems, an engagement order given by the driver, at the correct moment, is often also necessary.
A high structural outlay and considerable amount of space are therefore typical of all of these solutions. Their control of the differential lock is difficult, since the control also has to take account of the operating state of the differential-speed-dependent clutch. It thus has to be ensured, for example, that the axle differential is not locked if the differential-speed-dependent clutch is not yet in engagement.
As an alternative, WO-A 96/41090 (variant of FIG. 14) discloses driving the two wheels of the second axle individually, each via their own differential-speed-dependent clutch. In this case, a dedicated Gerotor pump is used for each side, referred to as a xe2x80x9ctwin arrangementxe2x80x9d. However, because of the much higher torque at the axle speed (than at the higher cardan shaft speed), said pump requires a very wide housing between the wheels of the second axle, this signifying a poor use of space and unfavorably short wheel shafts. Above all, however, the coordination and symmetrical distribution of torques to the two wheels and the interaction with electronically controlled brake systems (for example ABS) are not ensured. The distribution of torques also suffers due to the fact that during cornering the outer wheel on the curve runs in overrunning mode, i.e. does not contribute to the traction.
It is an object of the invention to propose a drive system which, with the smallest possible technical outlay, ensures that the differential lock responds in a manner coordinated neatly in terms of time and taking account of the driving state.
The foregoing object achieved according to the invention by a connecting line leading from the delivery side of the pump to an actuating member of the lockable differential, with the result that, when a differential speed occurs, the pressure produced by the pump makes it possible to at least partially lock the differential.
The differential-speed-dependent clutch therefore transmits the entire amount of torque intended for the second axle at a relatively high speed, and thus only needs a little amount of space. The actuation of the lock by the pressurized fluid from the pump of the differential-speed-dependent clutch saves having its own power source, permits the two locks to be readily coordinated in terms of time and can be adapted to the requirements of an electronically controlled brake system. It also ensures that the axle lock does not act before the differential-speed-dependent clutch is in engagement. With suitable dimensioning of the connecting line and adjustment of the actuating member, the differential can thus be locked at the correct moment or to the correct extent without a special control.
In a preferred embodiment, in which the entire differential-speed-dependent clutch automatically rotates together with the pump and is accommodated in a stationary housing, the operative connection to the actuating member of the lockable differential is produced via a rotary leadthrough between the rotating and stationary housings and via a connecting line to the actuating member. The fluid pressure is thereby transmitted into the housing and can be directed to the differential in a line fixed on the housing or even in the housing itself. When a controllable, differential-speed-dependent clutch is used, a rotary leadthrough is, as a rule, present in any case, with the result that additional outlay is not incurred.
In one possible embodiment, the pump or, for example if a fore-pump is present, at least one pump is arranged in the stationary housing from which the connecting line leads to the actuating member. The rotary leadthrough can therefore be omitted.
In one embodiment of the invention, the differential has a positive lock, the actuating member is fixed on the housing and acts on the lock via a mechanical power train (claim 4). In another embodiment of the invention, the differential has a positive lock, the actuating member of which is a concomitantly rotating fluid piston, and the operative connection to the pump is produced via a further rotary leadthrough. The first solution is particularly simple if a total lock of the axle differential satisfies the requirements and the second one is the better solution for sensitive adjustment of the locking torque.
In a development of the invention, a control valve for controlling the lockable differential is provided in the connecting line. This permits coordination in terms of time of the sequence of the locking of the differential-speeddependent clutch of the axial lock and the locking torques; but only the former in the case of a positive locking clutch. In the simplest case, the control valve is a throttle valve, however, it could also be a temperature-compensating valve, a pressure-limiting valve or a delay valve. To meet exacting requirements, it is a valve which is activated as a function of variables specific to the driving state.
To meet extremely exacting requirements in terms of driving dynamics, the differential-speed-dependent clutch can be controlled. For this purpose, the delivery side of the pump is connected via a pressure channel to a pressure space which is bounded by a piston acting on a friction clutch, and it being possible for the pressure acting on the piston to be controlled by means of a clutch valve which produces the connection between the delivery side of the pump and a low-pressure space by a discharge channel. In a particularly advantageous design of the invention, the connecting line branches off upstream of the clutch valve. As a result, the control of the differential-speed-dependent clutch also acts in the correct direction on the control of the differential lock. If the former control only responds weakly, the locking of the positive clutch also takes place with a more pronounced delay or the locking of the non-positive clutch also takes place only partially.
In a simpler design variant having a more simply constructed differential-speed-dependent clutch, in which the clutch valve is arranged in a discharge channel branching off from the pressure channel, the connecting line branches off upstream of the clutch valve. In this case, a single rotary leadthrough is sufficient for both valves without damage being done to the coordination of the two clutches.
In a developed design variant, in which the pump has a pressure space and an intake space and two discharge channels, one of which is connected, depending in each case on the direction of the difference in speed, to the intake space of the pump and one is connected to the pressure space, in which case the discharge channels are connected via a double clutch valve to the low-pressure space, the connecting line forms two branches, each of which is connected to one of the two discharge channels upstream of the clutch valve. The respectively other discharge channel then serves as an intake channel for the pump. A differential-speed-dependent clutch designed in such a manner is ABS-compatible. The two branches mean that the actuation of the differential lock is independent of the direction of the differential speed and is likewise ABS-compatible.
This developed design variant can be provided with a positive or with a non-positive differential lock. In the former case, a nonreturn valve is provided in each of the two branches and the control valve is a three-position valve (3/3-way directional control valve),
a) in the first position of which the connection between the pressure space of the pump and the actuating member is open,
b) in the second position of which the actuating member is connected to the low-pressure space, and
c) in the third position of which the actuating member is connected neither to the low-pressure space nor to the pressure space of the pump. In the position a) a lock is closed, in the position b) the lock is open, and in the position c) the lock is kept closed.
In the case of a non-positive locking clutch as differential lock, a control valve is provided in each of the two branches and is designed as a pulse-width-modulated control valve. The two control valves can be assigned to the direction of the differential speed and permit a particularly sensitive control of the differential lock.